Sputtering is a generally known technique for forming thin film. In the sputtering technique, a thin film is formed by sputtering a sputtering target. The sputtering technique is employed in industrial processes, since a thin film of large surface area can be readily formed, and a high-performance film can be formed with high efficiency. In recent years, various sputtering techniques have been known, such as reactive sputtering; i.e., sputtering in a reactive gas, and magnetron sputtering, which realizes high-speed thin film formation by placing a magnet on the backside of a target.
Among thin film products obtained through sputtering, indium oxide-tin oxide (In2O3—SnO2 compound oxide, hereinafter abbreviated to as ITO) film is a transparent conductive film which has high optical transparency with respect to visible light and high conductivity and which, therefore, finds a wide variety of uses such as for a liquid crystal display, a heat-generating film for defogging a glass panel, and an IR-reflecting film.
Thus, in order to produce thin films at higher efficiency and lower cost, modification and improvement of sputtering conditions and sputtering apparatuses are required and are now under way, and effective operation of sputtering apparatuses is essential. In the production of ITO film through sputtering, the period from setting of a new sputtering target to termination of initial arc (anomalous discharge); i.e., the period required for initiating formation of thin films, is preferably as short as possible, and assessing the sputter-enabling period of a target from the setting thereof (cumulative sputtering time: target life) is a key issue.
The aforementioned sputtering target for forming an ITO film is produced through mixing indium oxide powder and tin oxide powder at a predetermined ratio, molding under dry or wet conditions, and sintering the molded product (Patent Document 1). In this connection, highly-dispersible indium oxide powder has been proposed for producing high-density sintered ITO (see, for example, Patent Documents 2, 3, and 4).
Meanwhile, another known method includes sintering an ITO powder synthesized through the co-precipitation method under wet conditions (see, for example Patent Document 5). Similarly, a variety of wet-synthesis methods for producing ITO powder have been proposed for producing high-density sintered ITO (see, for example, Patent Documents 6 to 9).
Yet another method for producing an ITO powder containing an indium-tin-oxide solid solution phase in an amount of at least 90 vol. % in an indium oxide crystal lattice has been proposed. In the method, an indium-tin alloy is reacted with oxygen in plasma arc, followed by cooling the reaction product at a predetermined cooling rate or faster by means of a gas flow at a Mach number of ≧1 (see Patent Document 10), to thereby attain a predetermined resistivity of a green compact of the ITO powder.
However, even now, there is keen demand for an ITO powder which can readily produce a high-density sintered ITO without rigorously controlling sintering conditions and other conditions, thereby producing a long-life sputtering target.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 62-21751    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 5-193939    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 6-191846    Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 2001-261336    Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 62-21751    Patent Document 6: Japanese Patent Application Laid-Open (kokai) No. 9-221322    Patent Document 7: Japanese Patent Application Laid-Open (kokai) No. 2000-281337    Patent Document 8: Japanese Patent Application Laid-Open (kokai) No. 2001-172018    Patent Document 9: Japanese Patent Application Laid-Open (kokai) No. 2002-68744    Patent Document 10: Japanese Patent Application Laid-Open (kokai) No. 11-11946